This invention relates to an improved system for controlling the flow of vapor transported by a carrier gas from a bubbler to a using system. The control system is particularly useful in connection with high purity liquid source material used in the manufacturing of semiconductor devices.
The fabrication of semiconductor electronic devices includes many steps which require the transport of particular atoms or molecules to the surfaces of wafer substrates, usually maintained at elevated temperatures. In many of the steps, the most common method for accomplishing this is to transport the vapors from a liquid chemical source by a carrier gas stream into a reaction chamber of the using system. Consistent device performance depends strongly on accurate vapor delivery rates and extremely low levels of impurities, particularly metallics.
Typically, an ultra-high purity liquid source material is provided in a bubbler, and a suitable carrier gas stream is bubbled through the liquid and then transported to the point of use. The previous methods of vapor flow control that have been customarily used are the thermal-conductivity mass flow meter and the temperature controlled vaporizer bubbler; however, neither method has been entirely satisfactory.
The thermal conductivity mass flow meter monitors the vapor flow from a liquid source bubbler by taking the ratio of the thermal-conductivity of the carrier gas and vapor mixture flowing out of the bubbler, to the thermal-conductivity of the carrier gas flowing into the bubbler (see for example U.S. Pat. No. 3,650,151). The main drawback of this method is the introduction of metallic contamination in the vapor stream. The design and construction of the thermal mass flow meter has necessitated the use of metaliic parts, usually stainless steel, in the chemical vapor path. Because of the highly corrosive nature of many of the commonly used chemical vapors (especially in the presence of trace levels of moisture contamination) the metallic parts slowly deteriorate and the resulting metallic impurities are carried with the source vapor to the wafers. This leads not only to wafer contamination and low device yields, but also to drift and failure in the mass flow controller caused by the chemical deterioration. In addition the cost of the meters themselves is not a small problem in that they are costly, in the area of $2,000, and must be frequently repaired and replaced. This is particularly so with high carrier gas flow, such as in fiber optic applications.
The temperature controlled bubbler method maintains constant vapor mass flow by closely controlling the bubbler temperature and the mass flow rate of the carrier gas stream. Recently introduced improved bubblers, such as those illustrated in U.S. Pat. Nos. 4,134,514 and 4,140,735 eliminate many contamination and deterioration problems by using only high-purity quartz and teflon in contact with the vapors, and by eliminating chemical handling problems. The main drawback to this method has been fluctuations in the vapor mass flow such that the output has not always been sufficiently satisfactory and has required considerable trial and error adjusting of the carrier gas stream. As a result of inadequate controls, there is a significant and frequent loss of partially finished goods. In addition, there have been some reported instances of connections being broken or bubblers exploding because of improperly high pressures of the carrier gas streams. This results in danger to operating personnel because of the corrosive nature of the liquid source material, as well as the possible loss of the expensive work in progress, such as a batch of semiconductor wafers. Accordingly, a need exists for improved flow control of such a bubbler system. The present invention relates to such an improvement for the temperature controlled type bubbler system.